Generally, semiconductor devices include a plurality of circuits that form an integrated circuit (IC) fabricated on a semiconductor substrate. A complex network of signal paths will normally be routed to connect the circuit elements distributed on the surface of the substrate. Efficient routing of these signals across the device requires formation of multilevel or multilayered schemes, such as, for example, single or dual damascene wiring structures. The wiring structure typically includes copper, Cu, since Cu based interconnects provide higher speed signal transmission between large numbers of transistors on a complex semiconductor chip as compared with aluminum, Al, based interconnects.
Within a typical interconnect structure, metal vias run perpendicular to the semiconductor substrate and metal lines run parallel to the semiconductor substrate. Further enhancement of the signal speed and reduction of signals in adjacent metal lines (known as “crosstalk”) are achieved in today's IC product chips by embedding the metal lines and metal vias (e.g., conductive features) in a dielectric material having a dielectric constant of less than 4.0.
In semiconductor interconnect structures, electromigration (EM) has been identified as one metal failure mechanism. Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in microelectronics and related structures. As the structure size decreases, the practical significance of EM increases.
EM is one of the worst reliability concerns for very large scale integrated (VLSI) circuits and manufacturing since the 1960's. The problem not only needs to be overcome during the process development period in order to qualify the process, but it also persists through the lifetime of the chip. Voids are created inside the metal conductors of an interconnect structure due to metal ion movement caused by the high density of current flow.
Although the fast diffusion path in metal interconnects varies depending on the overall integration scheme and materials used for chip fabrication, it has been observed that metal atoms, such as Cu atoms, transported along the metal/post planarized dielectric cap interface play an important role on the EM lifetime projection. The EM initial voids first nucleate at the metal/dielectric cap interface and then grow in the direction to the bottom of the interconnect, which eventually results in a circuit dead opening.
FIGS. 1A-1D are pictorial representations of a prior art interconnect structure at various stages of an EM failure. In these drawings, reference numeral 12 denotes the dielectric cap, and reference numeral 10 denotes the metal interconnect feature; all other components of the prior art interconnect structure are not labeled to avoid obscuring the EM problem. FIG. 1A is at an initial stress stage. FIG. 1B is at a time when void 14 nucleation initiates at the metal interconnect feature 10/dielectric cap 12 interface. FIG. 1C is at a time when the void 14 grows towards the bottom of the conductive feature 10, and FIG. 1D is at a time in which the void 14 growth crosses the metal interconnect feature 10 causing a circuit dead opening.
It has been demonstrated that by replacing the Cu/dielectric interface with a Cu/metal interface can enhance electromigration resistance by greater than 100×. Prior art metal caps are typically comprised of a Co-containing alloy such as, for example, CoWP, which is selectively deposited atop of the Cu conductor region of the interconnect structure. One problem with utilizing such selective deposited metal caps is that some of the metal cap extends onto the adjoining surface of the interconnect dielectric material and, as such, electrical shorts between adjacent interconnects may arise. This is seen, for example, in FIG. 2 wherein reference numeral 20 denotes a dielectric material, reference numeral 22 denotes a conductive material embedded within the dielectric material 20, reference numeral 24 denotes a Co-containing alloy metal cap, and reference numeral 25 denotes metal residues from the Co-containing alloy cap process.
In addition to the above, it is known to provide a metal cap directly on the surface of an interconnect conductive material, such as, for example, Cu, by recessing the interconnect conductive material below a surface of the interconnect dielectric material. Such a structure is shown, for example, in FIG. 3. In this figure, reference numeral 20 denotes the interconnect dielectric material, reference numeral 22 denotes the interconnect conductive material embedded within the dielectric material 20, reference numeral 23 denotes a dielectric cap, and reference numeral 24 denotes the metal cap. Although this prior art recess process provides a metal cap that is located only on a surface of the recessed conductive material, such a process is problematic since there is high process cost associated therewith.
It is also worth mentioning that during a clean in dilute hydrofluoric acid, which is generally used to clean the surface of the interconnect dielectric material, corrosion of metal caps may occur. This is particularly observed when CoWP is used as the metal cap material.
In view of the above, there is a continued need for providing an interconnect structure in which the EM resistance is enhanced without degrading the time dependent dielectric breakdown reliability of the interconnect structure.